Generally, in a wireless communication apparatus for remotely operating a security device and the like such as the remote keyless entry system and like, mutual communication is carried out in a wireless manner between a portable device (transmitter), which is held by a vehicle user and has a transmitting and receiving function, and an in-vehicle device (receiver) including a communication control device having the same transmitting and receiving function. With such mutual communication, the door lock of the vehicle can be automatically locked and unlocked, and the engine can be started.
Such a wireless communication apparatus often uses a “multi-channel” method for mutual communication between the portable device and the in-vehicle device. In the multi-channel method, one frequency band is selected from multiple frequency bands prepared in advance to be used in the mutual communication. The frequency band to be used in the mutual communication is automatically switched each time the user operates the portable device. Therefore, even if the mutual communication at the selected frequency band is interfered due to degradation in a communication environment caused by noise and the like, the frequency band used in the mutual communication can be appropriately switched from a frequency band with large influence of noise to a frequency band with small influence of noise by switching the frequency band.
A periodic checking by the frequency band specified in advance is carried out between the portable device and the in-vehicle device to periodically check whether or not the portable device complies with the standard of the in-vehicle device.
Thus, the mutual communication, which uses the frequency band to be switched through input operation of the user, and the mutual communication, which uses a specific frequency band for periodic checking, are carried out between the portable device and the in-vehicle device. In other words, the portable device transmits a signal of an unknown frequency band switched through the input operation of the user and a signal of a known frequency band specified in advance to the in-vehicle device. Thus, it is difficult for the in-vehicle device to stably receive all the signals transmitted from the portable device. This leads to lowering in communication success rate between the portable device and the in-vehicle device.
A wireless communication apparatus described in Patent Document 1, for example, thus has been proposed to stably establish the mutual communication between the portable device and the in-vehicle device using multiple frequency bands. FIGS. 7 to 9 show an outline of the wireless communication apparatus.
As shown in FIG. 7, the wireless communication apparatus includes a portable device 10 having a wireless communication function, and a communication control device 20, which is mounted on a vehicle and used as a door lock/unlock device for performing a lock/unlock control of a door lock of the vehicle, for example.
The portable device 10 includes a mutual communication function for communicating with the communication control device 20. An operation command made by the user of the vehicle is input to an operation unit 13. The operation unit 13 transmits the input operation command to a control unit 11 serving as control means, which is a computer unit including a CPU, a ROM, a RAM, and the like. The control unit 11 thereby generates various operation signals such as a lock operation signal and an unlock operation signal for locking/unlocking the door lock of the vehicle in response to the operation command from the operation unit 13. The generated operation signal is output to a transmission unit 12. The transmission unit 12 modulates the operation signal input from the control unit 11 to a radio wave having a predetermined frequency, and transmits the modulated operation signal to outside via a transmission antenna 12a. The transmission unit 12 can modulate the operation signal to the radio wave of plural types of frequencies, and can change the transmitting frequency in accordance with a frequency control signal input from the control unit 11.
In the portable device 10 as described above, the control unit 11 successively transmits six data frames (first to six data frames) when transmitting the lock/unlock operation signal. In this case, the control unit 11 outputs a frequency switching signal to the transmission unit 12, so that the frequency channel is switched to a first channel or a second channel, and the lock/unlock operation signal can be transmitted over both channels.
As shown in FIG. 8, the transmission unit 12 includes two transmission circuits; a first transmission circuit 31 and a second transmission circuit 32. The transmission unit 12 further includes a first switch 33 arranged between the control unit 11 and the transmission circuits 31 and 32, and a second switch 34 arranged between the transmission antenna 12a and the transmission circuits 31 and 32. The first transmission circuit 31 modulates an operation signal input from the control unit 11 to a signal having a first frequency fa, and transmits the modulated signal to outside via the transmission antenna 12a. On the other hand, the second transmission circuit 32 modulates an operation signal input from the control unit 11 to a signal having a second frequency fb different from the first frequency fa, and transmits the modulated signal to outside via the transmission antenna 12a. The switches 33 and 34 switch between a connection state of “control unit 11-first transmission circuit 31-transmission antenna 12a” and a connection state of “control unit 11-second transmission circuit 32-transmission antenna 12a”. 
The communication control device 20 includes a communication control unit 21 serving as communication control means, which is a computer unit that includes a CPU, a ROM, a RAM, and the like, as shown in FIG. 7. The communication control device 20 has a reception antenna 22a for receiving the radio wave (lock/unlock operation signal) transmitted via the transmission antenna 12a. The lock/unlock operation signal received by the reception antenna 22a is input to a reception unit 22. The reception unit 22 demodulates the lock/unlock operation signal transmitted from the portable device 10 to a pulse signal, and outputs the pulse signal to the communication control unit 21. The communication control unit 21 performs a drive control of a door lock device 23 in accordance with the demodulated lock/unlock operation signal. The door lock of the vehicle is thereby locked/unlocked.
As shown in FIG. 9, the reception unit 22 includes a high frequency filter 61, a high frequency amplifier 62, an image cancelling mixer 63, an intermediate wave filter 64, an intermediate wave amplifier 65, and a demodulation circuit 66. These elements are connected in that order between the reception antenna 22a and the communication control unit 21. The reception unit 22 further includes a first local oscillator 68a capable of detecting the radio wave having the first frequency fa, and a second local oscillator 68b capable of detecting the radio wave having the second frequency fb. A switch 67 is connected between the first and second local oscillators 68a, 68b, and the image cancelling mixer 63. The switch 67 switches between a connection state of “image cancelling mixer 63-first local oscillator 68a” and a connection state of “image cancelling mixer 63-second local oscillator 68b”. In other words, the reception unit 22 can detect the radio wave having the first frequency fa in a state where the image cancelling mixer 63 and the first local oscillator 68a are connected, and can detect the radio wave having the second frequency fb in a state where the image cancelling mixer 63 and the second local oscillator 68b are connected.
In such a wireless communication apparatus, the lock/unlock operation signal is modulated to the radio wave of different frequencies such as the first frequency fa and the second frequency fb by switching the connection states of the switches 33 and 34 arranged in the portable device. In the communication control device 20, the connection state of the switch 67 is switched to receive the different frequencies, the first and second frequencies fa, fb, so that the first and second frequencies fa, fb are detected by the first and second local oscillators 68a, 68b, respectively. The portable device 10 then is capable of transmitting the radio wave of different frequencies selected in accordance with the communication environment, and the communication control device 20 is capable of receiving the radio wave having the selected frequency.